Conventionally, a seat belt device equipped in a vehicle such as an automobile prevents a vehicle occupant from being thrown out of a seat by securing the vehicle occupant using a seat belt to protect the vehicle occupant at the time of an emergency such as a case where rapid deceleration of a vehicle due to a collision occurs. Such a seat belt device includes a tongue slidably supported by the seat belt and a buckle fixed to a vehicle body or a vehicle seat. By inserting and engaging the tongue into the buckle, the seat belt is fastened across the vehicle occupant and the vehicle occupant sitting in the vehicle seat is secured and protected by the seat belt.
In order to detect whether the seat belt is fastened across the vehicle occupant, a buckle switch for detecting engagement between the tongue and the buckle is provided in the buckle. As a conventional buckle switch, there is provided a buckle switch in which an actuator for operating a press button of a switch is provided in a switch main body and which detects the engagement between the tongue and the buckle by moving a switch operator such as an ejector using a tongue inserted into the buckle, operating the actuator with the movement of the switch operator, and pressing the press button to switch on the switch with the operation of the actuator (for example, see Japanese Unexamined Patent Application Publication No. 2004-311091).
FIGS. 6(a)-6(c) illustrate a buckle switch disclosed in Japanese Unexamined Patent Application Publication No. 2004-311091, where FIG. 6(a) is a cross-sectional view illustrating an OFF state of the buckle switch, FIG. 6(b) is a cross-sectional view illustrating an ON state of the buckle switch, and FIG. 6(c) is a partial enlarged cross-sectional view of a contact portion of the buckle switch. In FIG. 6(a), 1 denotes the buckle switch, 2 denotes a switch main body, 3 denotes an actuator which is provided in the switch main body 2 such that it can be flexibly bent, 4 denotes a normally-closed fixed terminal fixed to the switch main body 2 (hereinafter, referred to as NC terminal), 5 denotes a normally-opened fixed terminal fixed to the switch main body 2 (hereinafter, referred to as NO terminal), 6 denotes a common terminal fixed to the switch main body 2 (hereinafter, referred to as COM terminal), 7 denotes a fixed contact electrically connected and attached to the NC terminal 4 at the upper side of the NC terminal 4, 8 denotes a fixed contact which is electrically connected and attached to the NO terminal 5 at the upper side of the NO terminal 5 and faces the fixed contact 7, 9 denotes a movable contact which can selectively contact two fixed contacts 7 and 8, 10 denotes a movable piece which is provided in the switch main body 2 such that it rotates about a point 10a and, electrically connects the movable contact 9 with the COM terminal 6, 11 denotes a second actuator which is provided in the switch main body 2 such that it rotates about a point 11a of one end thereof and has an operation point 1b at the other end thereof, 12 denotes a spring mounted between the movable piece 10 and the operation point 11b of the second actuator 11 in a pressed state, and 13 denotes a press button which contacts the operation point 11b of the second actuator 11 such that it can press the operation point 11b of the second actuator 11.
Furthermore, at the time of non-operation of the buckle switch 1, as shown in FIG. 6(a), the movable piece 10 is biased at the center of the point 10a in a clockwise direction by a biasing force of the spring 12, and the movable contact 9 of the COM terminal 6 contacts the fixed contact 7 of the NC terminal 4 and is separated from the fixed contact 8. Accordingly, the COM terminal 6 is electrically connected with the NC terminal 4 and disconnected from the NO terminal 5, and the buckle switch 1 is in the OFF state. At this time, since the movable piece 10 is biased by the biasing force of the spring 12 in a direction that the movable contact 9 contacts the fixed contact 7, the contact between the movable contact 9 and the fixed contact 7 is held without sudden separation.
In the non-operation state of the buckle switch 1, when a tongue (not shown) is inserted into a buckle (not shown), a switch operator (not shown) such as an ejector of the buckle is moved by this tongue. The actuator 3 is flexibly bent downward with the movement of the switch operator and presses the press button 13 downward. Then, the press button 13 is moved downward to contact the operation point 11b of the second actuator 11 and press the operation point 11b downward. Thus, the second actuator 11 rotates about the point 11a in the clockwise direction and the operation point 11b is moved downward. To this end, since the end of the spring 12 located at the side of the operation point 11b is moved downward, the biasing force of the spring 12 for biasing the movable piece 10 in the clockwise direction gradually decreases.
Furthermore, when the operation point 11b is moved downward and the tongue is inserted into a position where it can be engaged with the buckle, the direction of the biasing force of the spring 12 for biasing the movable piece 10 is changed from the clockwise direction to a counterclockwise direction and the movable piece 10 rotates about the point 10a in the counterclockwise direction. Thus, as shown in FIG. 6(b), the movable contact 9 is separated from the fixed contact 7 and contacts the fixed contact 8. Accordingly, the COM terminal 6 is disconnected from the NC terminal 4 and electrically connected to the NO terminal 5. As the result, the buckle switch 1 is switched on and is in an operation state. At this time, since the movable piece 10 is biased by the biasing force of the spring 12 in a direction that the movable contact 9 contacts the fixed contact 8, the contact between the movable contact 9 and the fixed contact 8 is held without sudden separation. In addition, since the contact is changed by the biasing force of the spring 12, the buckle switch 1 is switched from OFF to ON at a relatively high speed.
When the tongue is pulled out of the buckle, the force of the actuator 3 for pressing the press button 13 gradually decreases and finally disappears. Then, the second actuator 11 rotates about the point 11a in the counterclockwise direction by the biasing force of a return spring (not shown), and an operation portion C and the press button 13 are moved upward toward a non-operation position. The direction of the biasing force of the spring 12 against the movable piece 10 is changed from the counterclockwise direction to the clockwise direction and the movable piece 10 rotates about the point 10a in the clockwise direction. Then, as shown in FIG. 6(a), the movable contact 9 is separated from the fixed contact 8 and contacts the fixed contact 7. Thus, the COM terminal 6 is disconnected from the NO terminal 5 and electrically connected to the NC terminal 4. As the result, the buckle switch 1 is switched off and is in an initial operation state.
However, since the contact structure of the buckle switch 1 disclosed in Japanese Unexamined Patent Application Publication No. 2004-311091 is a butt contact structure in which the movable contact 9 vertically contacts each of the fixed contacts 7 and 8 the following problems are caused. That is, for example, as shown in FIG. 6(c), in a case where a gap C is generated between the movable contact 9 and the fixed contact 8, if a foreign material exists in the gap C, the foreign material is inserted between the movable contact 9 and the fixed contact 8 when the movable contact 9 contacts the fixed contact 8. Although not shown, similarly, even in a case where a gap C is generated between the movable contact 9 and the fixed contact 7, the foreign material is inserted between the movable contact 9 and the fixed contact 7. If the foreign material is inserted between the movable contact 9 and each of the fixed contacts 7 and 8, it may be difficult to sufficiently ensure electrical conduction. Furthermore, if a non-conductive product is generated at the respective surfaces of the fixed contacts 7 and 8 and the movable contact 9 due to a long-term use of the buckle switch 1, it may be difficult to sufficiently ensure electrical conduction.
Accordingly, it is difficult to surely operate the buckle including the buckle switch 1 and the seat belt device including the buckle for a long time, and it is difficult to stably perform various control operations related to a seat belt when driving a vehicle based on installation of the seat belt, such as an operation of controlling components of the vehicle and an operation of controlling tension of the seat belt.
In addition, since the buckle switch 1 has the butt contact structure, it is difficult to automatically remove a foreign material inserted between the movable contact 9 and each of the fixed contacts 7 and 8 and a non-conductive product generated at the respective surfaces of the fixed contacts 7 and 8 and the movable contact 9. Thus, a cleaning operation for removing the foreign material or the non-conductive product should be periodically performed.